This invention relates generally to the testing of integrated circuits, and, in particular, to probes for a fixed point probe card adapted to facilitate such tests.
Many circuits and complex multi-stage electronic systems that previously were regarded as economically unfeasible and impractical are now realizable with integrated circuits. The fabrication of a singlel-crystal monolithic circuit involves the formation of diodes, transistors, resistors and capacitors on a single microelectronic substrate. In practice, a microelectronic substrate for semiconductor integrated circuits is formed on a silicon wafer, the patterns for circuit processing being applied to the wafer by photolithography. Each wafer contains the patterns of many identical integrated circuits. After the patterns are tested, the wafer is sliced into "dice," each die containing a single circuit.
In an integrated circuit (IC) pattern, the input/output, power supply and other terminals of the circuit are formed by metallized contacts, usually deployed along the margins of the pattern. The outline of the pattern is either square or rectangular, and the marginal locations of the contact thereon depend on the circuit configuration and the available marginal space. Thus in a relatively simple circuit pattern, all of the marginal space may be available for contacts, whereas in more complex circuits, portions of the circuit may invade the marginal areas so that contact placement is restricted to the free marginal areas. In some instances, therefore, the contacts may lie in more or less uniform rows along the margins, and in other cases, the contacts may be randomly spaced from each other.
For the purpose of testing any type of integrated-circuit patterns, test probe cards have been developed and are now commercially available. Each test probe card consists of a printed circuit board having an opening to provide access to an IC pattern. The opening is surrounded by a ring of conductive pads connected by the printed circuit to card terminals for connection to test equipment appropriate to the circuit. The number of pads in the ring determine the maximum capacity of the probe card. If, for example, the card has forty-eight pads, it is capable of testing IC patterns having forty-eight contacts or less.
In setting up a probe card for a particular IC pattern, probes in the form of needles of tungsten are affixed to selected pads, the needles extending over the opening. The length and orientation of each needle is such that its point is adapted to engage a respective contact on the IC pattern.
Let us assume, by way of example, a very simple IC pattern lying within a rectangular outline and having five contacts at different marginal positions, such as two contacts adjacent on the upper and lower left corners of the rectangle and one contact adjacent to the lower right corners.
To assemble a probe card for this five contact, IC pattern, one must attach needles to those pads which are most conveniently located with respect to the contacts, and since the pattern is rectangular whereas the pads lie in a circular array, the stretch of each needle depends on the distance between its pads and the particular contact to be engaged. Hence the needles in a conventional probe card vary in length.
Since all of the contacts in the IC pattern lie in a common plane, and must be simultaneously engaged in order to carry out testing, it is essential that all needle points lie in a plane parallel to the common IC plane. Consequently, a fundamental requirement of a probe card is planarization of the needle points. The nature of conventional probe cards and the character of the assembly fixtures for setting up the probe positions for such cards are such that it is virtually impossible to assemble probes with needle points lying exactly in the same plane.